Theory of Illuminating Apparatus. By Dr. H. E. Fripp. 517 



sunlight (as from a cloud, whitened wall, &c.), or towards an 

 artificial light-source of sufficient extent of surface and from every 

 point of which an equally difiused light emanated (e. g. lamplight 

 with ground-glass plate in front of it or reflector behind it), no 

 other illuminator would be required for the greater number of 

 observations made. But on account of the mechanical arrange- 

 ments of the Microscope and for the convenience of the observer 

 a mirror is an indispensable addition, while for greater control 

 over the intensity of the light two accessory instruments are 

 desirable : namely, a diaphragm for exclusion and a lens for 

 collection (not condensation) of light. 



In discussing the action of the plane mirror as applied to the 

 Microscope, it must be borne in mind that from every light-source 

 having a greater area of surface than that of the mirror itself there 

 emanates a converging beam of light, which, after reflection, must 

 converge further to a given point in the axial line of the reflected 

 cone. If we hold a small mirror in the hand, with its surface turned 

 towards an unlimited expanse of bright sky, and so inclined to the 

 eye looking down upon it that the observer may see the reflected 

 sky, then, as the mirror is held further from or nearer to the eye, 

 a greater or smaller extent of sky is seen : or, if we look at a land- 

 scape, or a row of houses, or trees, &c., reflected from the mirror, 

 a greater or less number of objects is included in the field of vision 

 according to the distance of the mirror from the eye. And in 

 every case the field of vision is circumscribed by lines drawn from 

 the pupil to the periphery of the muTor and continued into space 

 in the direction of incidence. A trivial experiment ! but serving 

 to demonstrate that daylight illumination with the plane mirror 

 must always he effected with converging light. Each point in the 

 surface of the mirror reflects independently in every direction the 

 light it receives, and consequently when (in place of the eye 

 looking upon the mu-ror) we suppose an object to be placed on tlie 

 Microscope stage and in the optical axis of the instrument, the 

 whole mirror appears as a self-luminous surface in relation to that 

 object. Moreover, the luminousness of the surface remains equal, 

 whether the surface be curved or not (supposing of course the 

 light-source to be of practically unlimited extent). By reference 

 to Fig. 3 it will be seen that a converging incident beam on 

 the plane mirror is reflected on the same focal point as the parallel 

 beam thrown on the concave mirror ; and that a certain inclination 

 of the mirror (supposing it to be centered on the optical axis of the 

 instrument and arranged for direct central illumination) of 45^ is 

 necessary in order to reflect this converging beam on the object. 



When a parallel beam of light is reflected from the plane 

 mirror set at 45° to the axis of the instrument, the central rays 

 alone fall on the object and the illumination is proportionally 



